Usually these bottles are made from a cylindrical preform which, after suitable heating, is introduced into a mould and subjected in succession to stretching and blowing steps.
In PET bottle filling processes, the bottles which have not yet been filled must have a form such as to withstand the stresses due to the movement along the entire production line. Subsequently, when they are filled, they must be able to withstand the stresses arising during the palletization and transportation operations.
For this reason, the bottles are designed with geometric shapes such as to ensure the best possible mechanical performance and avoid excessive deformation.
However, the mechanical performance is not the only factor which must be taken into consideration.
Should the shape of the bottle, in order to achieve a satisfactory mechanical strength, be very complex, the associated production process might not be sufficiently simple. In particular, the parison blowing operation might not be able to ensure an adequate degree of reproducibility of the bottle, in particular in the region of the base.
In other words, a bottle with a geometric shape which is highly resistant to mechanical stresses could be too complicated to produce by means of the stretching and blowing process. Therefore, the experience of the polymer bottles designer, attempts to achieve a suitable compromise between rigidity of the container due to the geometry, empty weight of the bottle (i.e. quantity of material to be distributed in the stretching and blowing process) and “blowability” of the container, understood as meaning the capacity for reproducing a given form while maintaining a more or less constant thickness of the material throughout the bottle.
Moreover, manufacturers tend to gradually reduce the weight of the bottles with the aim of achieving a saving in material costs and a reduction in the environmental impact.
In the case of water containers, the low value of the contents of the bottle make it even more desirable to achieve a reduction in weight of the container. This has resulted in increasingly smaller wall thicknesses down to minimum thickness values of the polymer material forming the bottle ranging between 0.05 and 0.3 mm.
When the bottle is filled with liquid such as water a problem arises in connection with handling thereof, namely compression of the walls in the radial direction and axial direction, which compression is even more pronounced when there is a reduction in the thickness of the walls.
In order to make the bottle stronger and more resistant to the aforementioned deformations, one solution which has been proposed and implemented is that of adding nitrogen in the liquid state to the contents of the bottle inside the space at the top of the container immediately after the filling step and just before the capping step. The nitrogen evaporates and expands inside the empty space between the liquid and the cap. The bottle is thus pressurised and is able to withstand greater axial and radial load stresses than a bottle without nitrogen.
This technology is applied in particular when filling, with water or other liquids which have not been added with gas, bottles having a weight which is very light and unable to ensure per se an adequate mechanical performance.
Pressurisation of the bottle, however, creates stresses in some cases such that they deform excessively the bottle and in particular its base which, if not sufficiently strong, may flex or bow outwardly.
Outward flexing of the base results in the instability of the bottle which does not rest over its normal supporting area, but on the central point of the base, making the bottle unstable with the evident problems in terms of both transportation and use and handling by the end user.
At the state of the art there exist other methods of pressurisation which are implemented using mixed sterile compressed air or carbon dioxide used in the case of gaseous beverages. In the case of these applications, also, the improvements and problems are similar to those encountered when performing filling with nitrogen.
Therefore, the prior art, although widely established, is not without drawbacks.
In fact, hitherto the geometry of the bottle base has been designed to optimize the structural resistance to high internal pressures, using the conventional solution of increasing the rigidity of the base by means of ribs, the centre of which, projected along the supporting surface, has a radial progression. In this connection, FIG. 1 shows the base of a bottle according to the prior art. The term “rib” is understood as meaning a curvature of the base of the bottle which forms a protuberance in the longitudinal direction, directed towards the inside of the bottle, so that they appear to the observer as recesses.
Moreover, in order to increase the resistance to stresses due to pressurisation with nitrogen or other pressurisation systems, it has been attempted to increase the number and depth of the radial ribs, without however achieving the expected success both for structural reasons and in particular because the moulding operation is very complex.
The internal pressure tends to deform the base, causing it to assume a form which resembles most closely (at the theoretical limit values) the form of a semi-sphere. The line of extension of the conventional rib would therefore tend to assume the form of a cord lying on a sphere and joining the outermost point of the base with the central point of the base.
In the conventional solutions the internal pressure tends to deform the base which withstands in the inertia cross-section perpendicular to the radial direction only moments contained in radial planes comprising the longitudinal axis of the bottle.